One scheme replaces the original loans with new tax-free loans. In emails to a potential client, a promoter claims that the new loans would not need to be declared to HMRC.

Mr Thomas said this risked doubling the tax charge, as HMRC could seek tax on the new loans as well as the original outstanding loans.

He added: “Even if the scheme does successfully mitigate the loan charge, you still need to deal with the original inquiry and tax liability.”

The tax office said it had stopped several promoters in the past two years and forced about 30 to hand over details of their schemes.

It has also successfully reported three companies to the advertising watchdog for making misleading claims over tax.

A spokesman said: “Any loan repayments connected to one of these tax avoidance arrangements will be ignored and the loan charge will still apply, despite [taxpayers] being left out of pocket by promoter fees."